Transforming Growth Factor-β Messenger RNA and Protein in Murine Colitis

Using a CD4+ T-cell-transplanted SCID mouse model of colitis, we have analyzed TGF-β transcription and translation in advanced disease. By in situ hybridization, the epithelium of both control and inflamed tissues transcribed TGF-β1 and TGF-β3 mRNAs, but both were expressed significantly farther along the crypt axis in disease. Control lamina propria cells transcribed little TGF-β1 or TGF-β3 mRNA, but in inflamed tissues many cells expressed mRNA for both isoforms. No TGF-β2 message was detected in either control or inflamed tissues. Immunohistochemistry for latent and active TGF-β1 showed that all cells produced perinuclear latent TGF-β1. The epithelial cell basal latent protein resulted in only low levels of subepithelial active protein, which co-localized with collagen IV and laminin in diseased and control tissue. Infiltrating cells expressed very low levels of active TGF-β. By ELISA, very low levels (0–69 pg/mg) of soluble total or active TGF-β were detected in hypotonic tissue lysates. TGF-β1 and TGF-β3 are produced by SCID mouse colon and transcription is increased in the colitis caused by transplantation of CD4+ T-cells, but this does not result in high levels of soluble active protein. Low levels of active TGF-β may be a factor contributing to unresolved inflammation.

Abnormal mucosal extracellular matrix deposition is associated with increased TGF-beta receptor-expressing mesenchymal cells in a mouse model of colitis

Transforming growth factor-beta (TGF-beta) depresses mucosal inflammation and upregulates extracellular matrix (ECM) deposition. We analyzed TGF-beta receptors RI and RII as well as ECM components using the CD4(+) T-cell-transplanted SCID mouse model of colitis. The principal change in colitis was an increased proportion of TGF-beta RII(+) mucosal mesenchymal cells, predominantly alpha-smooth muscle actin (SMA)(+) myofibroblasts, co-expressing vimentin and basement membrane proteins, but not type I collagen. TGF-beta RII(+) SMA(-) fibroblasts producing type I collagen were also increased, particularly in areas of infiltration and in ulcers. Type IV collagen and laminin were distributed throughout the gut lamina propria in disease but were restricted to the basement membrane in controls. In areas of severe epithelial damage, type IV collagen was lost and increased type I collagen was observed. To examine ECM production by these cells, mucosal mesenchymal cells were isolated. Cultured cells exhibited a similar phenotype and matrix profile to those of in vivo cells. The data suggested that there were at least two populations of mesenchymal cells responsible for ECM synthesis in the mucosa and that ligation of TGF-beta receptors on these cells resulted in the disordered and increased ECM production observed in colitic mucosa.

Processing of the transforming growth factor beta type I and II receptors. Biosynthesis and ligand-induced regulation

Three cell surface transforming growth factor beta (TGFbeta) receptor (R) proteins regulate the effects of TGFbeta isoforms on growth and differentiation. TGFbeta-IR and -IIR are transmembrane serine/threonine kinases directly mediating the signaling across the plasma membrane. Both TGFbeta and its receptors are ubiquitously expressed, hence the fine regulation of the multiplicity of responses most likely involves several levels of control including the regulation of expression, complex formation, and down-regulation of the receptor proteins. In mink lung epithelial cells, TGFbeta-IIR was first synthesized as a approximately 60-kDa endoglycosidase H-sensitive precursor protein, which was converted to a mature approximately 70-kDa protein. The half-life of metabolically labeled mature TGFbeta-IIR was estimated to be 60 min and was further reduced to approximately 45 min in the presence of exogenous TGFbeta1. Minimal internalization of 125I-TGFbeta1 at 37 degrees C was detected suggesting that the rapid turnover was not due to endocytosis and degradation of the ligand-receptor complexes. TGFbeta-IR was synthesized as a approximately 53-kDa precursor protein, which was processed to a mature approximately 55-kDa receptor protein. The half-life of TGFbeta-IR was >12 h. A fraction of tunicamycin-treated type I and II receptors that reach the cell surface was able to associate in the presence of ligand suggesting that heteromeric complexes can form in a post-endoplasmic reticulum compartment before full glycosylation is achieved. These results show differential processing and turnover of TGFbeta-IR and TGFbeta-IIR providing a potential additional mechanism for modulation of cellular responses to TGFbetas.